Process of producing calcium by electrolysis



United States Patent 3,226,311 PROCESS 0F PRUDUCHNG CALCEUM BY ELECTRQLYSHS Jacques Van Diest, Uccle-Brussels, Beigiurn, assignor to Solvay & Cie, Uccle-Brussels, Belgium No Drawing. Filed May 28, 1963, Ser. No. 283,694

Claims priority, application Belgium, May 13, 1959, 458,486

7 Claims. (til. 204-69) This is a continuation-in-part of my copending application, Serial No. 25,530 filed April 29, 1960, and now abandoned.

The present invention relates to a new process for the preparation of calcium by electrolysis of baths of fused salts, and it is also concerned with a process for preparing alloys of calcium with metals which are less noble from the electrochemical point of view.

The usual process for the preparation of calcium by electrolysis in the fused state is the process which produces calcium rods. In this process, the calcium is deposited in the liquid state at the cathode and solidifies as it is extracted from the bath of fused salts in the form of a rod or core. The conditions necessary for good formation of the core are extremely critical. Thus, one uses a bath of fused calcium chloride in the pure state, or alloyed with a lesser quantity of calcium fluoride, and to maintain this bath in the fused state it is necessary to use temperatures of the order of 800 C. The electrolysis of calcium chloride requires, under these conditions, a voltage in the region of volts and a cathode current density which is very high, e.g. above 100 amperes per square centimeter. The disadvantages of such a process are apparent; it requires a high consumption of power not only for the electrolysis proper, but also to obtain and maintain the bath of salts in the fused state. This consumption is of the order of 50 kilowatt per kilogram of calcium product. Moreover, the elevated temperatures required for carrying out the process lead to considerable corrosion of the materials of construction of the cell and reduce the life of the latter. Furthermore, chlorine is obtained at the anode in a very diluted state and, finally, the process is essentially discontinuous and requires considerable labor.

I have now discovered that it is possible to obtain calcium in a pure state by the electrolysis of baths of fused salts and under conditions of current efficiency and power consumption which are extremely advantageous.

I have established the surprising fact that by using as baths of fused salts, rather than the chloride of calcium alone or admixed with a little calcium fluoride, the chloride admixed with chlorides of strontium and barium in the proportions of 20 to 80 mol percent of calcium chloride, 5 to 45 mol percent of strontium chloride and 5 to mol percent of barium chloride. The bath can also contain 0 to 35 mol percent of potassium chloride. Preferably the proportions are to 75 mol percent of calcium chloride, 15 to 35 mol percent of strontium chloride, and 15 to 30 mol percent of barium chloride and, when potassium chloride is used, at least 10 mol percent of this salt is preferably present, with at least 25 mol percent being particularly preferred. Electrolysis is carried out at a temperature of 650 to 750 C. and the calcium is produced in the solid state and in a very pure form at the cathode. One thus obtains directly the calcium in a compact form by operating in the presence of a cathode, preferably rotating, constituted by a metal which forms no alloy with calcium. Moreover, it is possible to render the process continuous by arranging a doctor blade at the cathode which removes the calcium deposited.

The process forming the object of the invention is therefore characterized by the fact that one subjects to electrolysis a bath of fused salts which comprises, besides the chloride of calcium, at least strontium chloride and barium chloride, and preferably also potassium chloride, in the proportions indicated at a temperature between 650 and 750 C. and under a voltage of 5-7 volts, the calcium being obtained directly in the solid compact state on a rotating cathode of a metal which forms no alloy with calcium. Further characteristics of the invention become apparent from the following.

Among the metallic ions which may constitute the bath of fused salts, sodium, being slightly more noble from the electrochemical point of view than the alkaline earth metals, is suitably present as an ion in small quantities, e.g. 5 to 15%. Furthermore, it goes without saying that the process proposed makes it possible to obtain alloys of calcium with metals more noble than they are, that is to say, of alkali metals, by a judicious choice of the constituents of the baths of fused salts.

The advantages of the new process are many, above all by reason of the decrease of the temperature at which one operates. Thus the power required for obtaining and maintaining the salt bath at the electrolysis temperature is reduced. The corrosion of the material of construction of the cell is reduced and, in consequence, the life of the latter is prolonged. The solubility of the metal in the bath is further reduced, which avoids losses by redissolving of the metal, and likewise losses by vaporization of the metal are avoided because of the reduction of its volatility. The process permits operation with voltages in the region of 5 to 7 volts instead of 25 volts and the power consumption expressed in kilowatt hours per kilogram of alkaline earth metal produced is reduced to one-fifth of the consumption observed in the usual process. In addition, the process permits operation at relatively high current intensities, e.g. amperes. This is a particularly important advantage since operation at such current intensities with processes of the prior art have involved a serious sacrifice of efiiciency. High current intensities permit, of course, a greater production for a given apparatus.

The new process is carried out in a cell constituted by a vessel made of generally impermeable refractory material serving to contain the bath of fused salts, a graphite anode and a cathode made of a metal which forms no alloy with calcium. The process is preferably carried out in an apparatus such as that described in Belgian Patent 578,670 under the title Process for the Preparation of Alkaline Earth Metals and of Alloys Based on Alkaline Earth Metals by Electrolysis of Baths of Fused Salts, wherein the anode is of an annular form and a screen of insulating and impermeable material imposes a non-linear course on the current lines between the electrodes. The cathode slowly turns, for example at 50 revolutions per minute, which avoids the formation of metal in the form of dendrites and favors the coalescence of the metallic deposit on the cathode. The metal collected is compact and homogeneous. By contrast to the hitherto known electrolytic processes, the new process enables the calcium to be obtained in a very pure form, the degree of purity being above 96%. The oxygen content of the product obtained is much smaller than that of the product obtained by the rod or core process.

The yield is further improved by subjecting the baths of fused salts to a previous purification for the purpose of removing all traces of water and oxygen. For this purpose, hydrogen chloride in the dry state or chlorine is bubbled into the bath of fused salts. This treatment may be advantageously combined with a pre-electrolysis under reduced voltage for example of 2 to 3 volts.

During the course of electrolysis it has been established that the current density is not critical. It may be very small without affecting the current efficiency which permits working with an extremely low power consumption.

Another feature of the present invention is the possibility of preparing alloys of calcium with metals which are more noble from the electrochemical point of view. For this purpose, a cathode is used which is made of the metal which it is desired to be alloyed with the calcium. In this way there may be obtained alloys by using a cathode formed of magnesium, aluminum, silver, copper, gold, etc.

To prepare calcium according to the present invention, fused baths of the specified composition are subjected to electrolysis. The following compositions, expressed in gram mol percent, are particularly suitable for carrying out the manufacture of calcium according to the process which forms the object of the present invention:

CaCl 60 SrCl 20 BaCI 20 CaCl 25 SrCl 25 BaCI 25 KCl: 25

These compositions permit an operation at a temperature below 700 C. comprised between 660 and 680 C.

The following examples serve to illustrate the advantages of the process forming the objects of the invention. It must be understood that they do not limit the scope of the invention which is capable of variations without departing from the invention as defined in the claims.

Example 1 There is subjected to electrolysis a bath of fused salts comprising 60 gram mol percent calcium choride, 20 gram mol percent strontium chloride, and 20 gram mol percent barium chloride previously purified by treatment with dry hydrogen chloride gas or chlorine. A small quantity of alumina (A1 e.g. up to 3% by Weight of the bath, preferably at least 0.2% is added in powder form to improve the stability of the electrolysis.

The bath temperature is maintained at 676 C. and, in the presence of a stainless steel cathode rotating at 50 revolutions per minute, electrolysis is carried out under a voltage of 5.33 volts. The cathode current density is 2 amperes per square centimeter.

At the cathode, solid calcium is collected in a compact and homogeneous form. It is brilliant in appearance and its degree of purity exceeds 96%. The impurities are principally formed by strontium and barium.

It has been found that by operating in this manner the anode current efliciency is 80% and the power consumption is 9 kilowatt hours per kilogram of calcium, instead of 50 kilowatt hours per kilogram of calcium as in the conventional process.

Example 2 At a temperature of 670 C. a bath of fused salts is subjected to electrolysis, formed by 25 gram mol percent calcium chloride, 25 gram mol percent strontium chloride, 25 gram mol percent barium chloride and 25 gram mol percent potassium chloride at a voltage of 5.29 volts and an anode current density of 2 amperes per square centimeter.

Calcium is obtained in a solid homogeneous and compact state with an anode current efficiency of 79%. The power consumption is 9 kilowatt hours per kilogram of calcium.

Example 3 Example 1 is repeated but working at 660 C. and with a current density of 2.5 amperes per square centimeter. Calcium is obtained in more than 96% purity with a power consumption of 9 kilowatt hours per kilogram of calcium.

Various other combinations of salts falling within the ranges set forth above were subjected to electrolysis at various temperatures. These additional examples of the process of this invention are set forth below. In the 4- following table, the concentrations of the salts are given in mol percent:

Example Melting Electrolysis No. CaCl SrCl BaCI K01 ppigt, temperature Example 13 A bath containing 60 gram mol percent of CaCl 20 gram mol percent of BaCl and 20 gram mol percent of SrCl and having a melting point of 608 C., was submitted to electrolysis for an hour and a half at a temperature of 675 C. There was obtained 108 g. of calcium metal of 98 percent purity. The current efiiciency both for chlorine and calcium amounted to percent.

In th table below the conditions of the foregoing example are set forth.

Current (amperes) 100 Temperature (C.) 675 Period of electrolysis (minutes) 90 Current density (amp/cm?) 8 Voltage 5.37 Ampere hours Chlorine current eificiency (percent) 95 Chlorine evolved (grams) 167 Calcium current etficiency (percent) 95 Calcium collected (grams) 106 Example 14 A bath containing 50 gram mol percent of CaCl 15 gram mol percent of SrCl and 35 gram mol percent of BaCl at a temperature of 720 C. was submitted to electrolysis under a voltage of 5.05 volts and a current of 20 amperes. The current efficiency both for chlorine and calcium was 95 percent.

Example 15 A bath containing 60 gram mol percent of CaC1 20 gram mol percent of BaCl 20 gram mol percent of SrCl and 3 weight percent of A1 0 was submitted to electrolysis at 670 C. under a voltage of 5.10 volts and a current of 12 amperes. The current efficiency both for chlorine and calcium was 98 percent.

I claim:

it. A process for the production of calcium metal by electrolysis of a bath of fused chlorides at a temperature lower than the melting point of calcium, which comprises subjecting to electrolysis a bath comprising 20 to 80 mol percent of calcium chloride, 5 to 45 mol percent of strontium chloride, 5 to 35 mol percent of barium chloride and 0 to 35 mol percent of potassium chloride, said electrolysis being carried out at a temperature between 650 and 750 C.

2. A process as defined in claim 1, wherein the bath contains 40 to 75 mol percent of calcium chloride, 15 to 35 mol percent of strontium chloride, and 15 to 30 mol percent of barium chloride.

3. A process for the preparation of calcium which comprises subjecting to electrolysis a bath of fused salts comprising 60 gram mol percent calcium chloride, 20 gram mol percent strontium chloride, and 20 gram mol percent barium chloride at a temperature between 660 and 680 C. and at a voltage of 5 to 7 volts, said electrolysis being carried out in the presence of a cathode of a metal which forms no alloy with calcium, the calcium being obtained directly in the solid compact state on said cathode.

4. A process for the preparation of calcium which comprises subjecting to electrolysis a bath of fused salts comprising 25 gram rnol percent calcium chloride, 25 gram mol percent strontium chloride, 25 gram rnol percent barium chloride and 25 gram mol percent potassium chloride at a temperature between 660 and 680 C. and at a voltage of 5 to 7 volts, said electrolysis being carried out in the presence of a cathode of a metal which forms no alloy with calcium, the calcium being obtained directly in the solid compact state on said cathode.

5. A process as defined in claim 1, wherein said bath further comprises up to 3% by weight of the bath of alumina.

6. A process as defined in claim 1 wherein during said References Cited by the Examiner UNITED STATES PATENTS 886,857 5/1908 Price et al. 204-69 2,850,442 9/1958 Cathcart et al. 204-68 3,043,756 7/1962 Cobel et al. 204-69 JOHN H. MACK, Primary Examiner. WINSTON A. DOUGLAS, Examiner. 

1. A PROCESS FOR THE PRODUCTION OF CALCIUM METAL BY ELECTROLYSIS OF A BATH OF FUSED CHLORIDES AT A TEMPERATURE LOWER THAN THE MELTING POINT OF CALCIUM, WHICH COMPRISES SUBJECTING TO ELECTROLYSIS A BATH COMPRISING 20 TO 80 MOL PERCENT OF CALCIUM CHLORIDE, 5 TO 45 MOL PERCENT OF STRONTIUM CHLORIDE, 5 TO 35 MOL PERCENT OF BARIUM CHLORIDE AND 0 TO 35 MOL PERCENT OF POTASSIUM CHLORIDE, SAID ELECTROLYSIS BEING CARRIED OUT AT A TEMPERATURE BETWEEN 650 AND 750*C. 